1. Field of the Invention
The present invention relates to a DTMF signal receiving circuit and, more particularly, to a circuit structure which effectively extracts a DTMF signal from a received signal transmitted from a telephone line.
2. Description of the Related Art
In home telephones having an answering function, it is possible to remote-control various devices by operating them from an external telephone. This remote control is achieved by receiving a DTMF (Dual Tone Multiple Frequency) signal.
FIG. 6 shows a circuit diagram of an answering machine. A ringer circuit 12 and a speech network circuit 14 for adjusting the level of a voice signal or the like are connected to a hook switch 10. A DTMF signal receiving circuit 16, a recording/reproducing circuit 18 for recording and reproducing the voice information received by the speech network circuit 14, and a dial circuit 20 are connected to the speech network circuit 14, and these circuits are controlled by a controller 22.
The DTMF signal receiving circuit 16 detects a DTMF signal on the basis of the signal received from the speech network circuit 14, decodes the received DTMF signal, converts the signal into data of 4 bits or the like, and outputs the data to the controller 22. For example, when remote-controlling an answering machine or the like, an identification number consisting of DTMF signals is supplied from an external telephone or the like, and a remote control signal composed of DTMF signals is then supplied. The controller 22 can therefore operate the recording and reproducing operations on the basis of the remote control signal. It is also possible to control home appliances or a home security system by outputting the remote control signal to various devices connected to the telephone.
In such a DTMF signal receiving circuit, for example, when a user switches off the loop of the telephone line by the on-hook operation or the like after supplying the identification number and before supplying the remote control signal, a monitoring signal, what is called a dial tone signal, which shows the connecting state between the telephone and the telephone exchange is supplied, which sometimes causes malfunctions of various devices. Why such malfunctions are caused will be explained in detail in the following.
It is now assumed that the dial tone signal is composed of fundamental frequency components of 400 Hz. The width of the fundamental frequency component varies depending on the various conditions of the telephone line or the like. When the fundamental frequency component has a frequency of 395 Hz as shown in FIG. 7, for example, higher harmonic components which are twice and three times as high as the fundamental frequency appear in the DTMF signal receiving circuit 16. As shown in FIG. 7, the higher harmonics have a frequency of 790 Hz and 1185 Hz, and these higher harmonic signals exist in the frequency band of the DTMF signal. The DTMF signal receiving circuit 16 detects the higher harmonic signals which are similar to the DTMF signal, and these higher harmonic signals cause malfunctions.
In order to solve the problem, the device described in Japanese Patent Laid-Open No. Hei 4-200157, for example, is provided with a means for detecting a dial tone signal from the received signal so that the device doesn't decode the DTMF signal when detecting the dial tone signal.
Although the DTMF signal is standard throughout the world, the dial tone signal varies depending on the country. Within Japan, different dial tone signals are used. For example, Nippon Telegraph & Telephone Public Corp. uses a dial tone of 400 Hz and DDI Corp. uses a dial tone of 900 Hz. However, since the conventional dial tone signal detecting circuit can only detect the dial tone signal having one kind of frequency, it is necessary to provide a dial tone signal detecting circuit in accordance with the frequency of the dial tone signal of the user.